Method and apparatus for reducing echo effects in picture transmission systems



Feb. 13, 1945. p, B, rr v 2,369,614- METHOD AND APPARATUS 'FOR REDUCING ECHO EFFECTS IN PICTURE TRANSMISSION SYSTEMS v v Filed llay 5, 1942 SSheets-SheQt 1 j 611/0460: v 15am 516/6410.

30/71/7dA/0 Q mums-o K014130036! IMP j D. B. SMITH Feb. 13, 1945.

METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS IN PICTURE TRANSMISSION SYSTEMS 3 Sheets-Sheet 2 Filed May 5. 1942 Si n. 5S0

Feb. 13, 1945. D. B. SMITH 2,369,l4

METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS IN PICTURE TRANSMISSION SYSTEMS Filed May 5, 1942 5 Sheets-Sheet 5 Patented Feb. s, 1945 METHOD AND APPARATUS FOR REDUCING ECHO EFFECTS IN PICTURE TRANSMIS- SION SYSTEMS David B. Smith, Philadelphia, Pa., assignor to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application May 5, 1942, Serial No. 441,374

r24 Claims. (01. 178-71) This invention relates to picture transmission systems and the like, and to a method and means for substantially reducing or eliminating the effects of echo signals on the received or reconstituted picture. More particularly, the invention relates to television systems, and to an improved method and means for reducing or substantially eliminating certain undesirable effects resulting from the arrival, during the picture line periods, of echoes of the horizontal or line synchronizing signals.

One of the problems encountered in picture or television transmission, and one that obtains with any type of modulation, is that which results from the reception of long delayed echoes, corresponding to signal path difierences of the order of a mile or several miles. Since the speed of propagation ofv a radio wave is approximately one thousand feet per microssecond, a path difference of say three orfour miles, as between the direct and the reflected signal path, may produce echoes delayed by or micro-seconds. In magnitude, these echoes may be relatively faint due to the greater path length, to the fact that the echosignal normally sufiers considerable attenuation in reflection, and. to the fact that the indirect path is usually closer to the earth than is the direct path. On the other hand, since the synchronizing signals are of greater amplitude than the line or picture portions of the signal, reflected synchronizing signals of considerable strength sometimes appear in the picture as a result of these long-delayed echoes. In generahan echo may appear in a television picture by inbining or beating with the picture carrier. Thus the echo may add to or subtract from the picture carrier depending upon the particular phase relation between the two signals at a given instant. This phase relation depends (except for changes effected at the transmitter itself) upon the difference in path length between the direct and reflected signals; and the only way in which this difference can be appreciably varied,-assuming that the transmittingand receiving antennas are fixed, is by a variation in the placement of the reflector causing the echo. But since the reflector is usually a high building, bridge, gas tank, hill, or similar object,it too can be regarded as a substantially fixed structure, and hence in any given installation the echo signal, as it appears upon the viewing screen at the receiver', is subject only to such phase relations between the direct and reflected signals as may result from periodic phase changes effected within the transmitter itself.

joint application of F. J. Bingley and W. E. Bradley, Serial No. 433,660, filed March 6, 1942, and the copending application of F. J. Bingley, Serial No. 435,402, filed March 19, 1942, this can be accomplished most readily by periodically changing I the phase of the echo carrier with respect to the picture carrier, these changes being so timed that the successive echoes balance each other out so fares their impression upon the observer is concerned, Thephase changes referred to are effected at the transmitting station and require no additional equipment at'the point of reception.

' The present invention utilizes the broad principles disclosed in the above-named copending joint application, but an improved method is utilized to eflectthe changes in phase between the echo carrier and the picture carrier. In the method described by Bingley and Bradley, the carrier phase changes are effected suddenly almost instantaneouslythrough the agency, for example,of an electronic phase-reversing switch. Now while the Bingley-Bradley method has proven satisfactory in practice, it is possible, under certain conditions, that such method may prov duce short-time amplitude transients whichmay be difficult to eliminate. According to the present invention, the phase of the carrier is advanced, retarded, or reversed, gradually by changing the carrier frequencya predetermined amount for a predetermined time interval until the carrier has lost or gained the desired number of electrical degrees degrees in a phase reversal system). 'When the carrier phase is changed in this manner, the carrier amplitude may remain substantially constant and consequently no amplitude transients are produced.

It is a principal object of this invention to provide amethod and means for substantially eliminating certain echo eflectswhich may be encountered in picture transmission systems.

Another object of the invention .is to provide an improved method and means for transmitting a television signal, or the like, which when received will produce a picture which is substantially free'of echoes of the synchronizing signals.

Still another object of the invention is to provide a method and means by which the relative Phase of echo signals may be periodically advanced or retarded without causing amplitude pable of generating my improved echo-cancelling carrier signal.

These and other objects and features of the invention will be apparent from the following description andthe accompanying drawings in which:

Figs. 1 and 2 are explanatory diagrams in connection with which the method of the present invention will be explained;

' Fig. 3 is a block diagram of a television transmitter, constructed in accordance with the invention, for generating the improved televisio signal; and

Fig. 4 is an explanatory diagram illustrating the operation of thetelevislon system of Fig. 3.

In Fig. 1 there is represented the amplitude and frequency characteristics of a television carrier wave modulated in accordance with the principles ofthe present invention. The portions shown comprise the blanking interval A and "La part of the video interval B of a given picture line, and the blanking interval A and part of the video interval B for the same line in the succeeding frame. In a 525-line, 30-frame television system the signal AB and the signal A'B' are separated in time by a 525-line interval ta of substantially one-thirtiethof a second. The upp r portion of Fig. 1, which is plotted against an amplitude scale, may be regarded as a representation of the positive half of the carrier wave envelope. Since the negative half of the envelope is identical in appearance, it has been omitted in this illustration. The blanking and synchronizing portion of the signal which occupies the interval A, comprises the synchronizing signal i of duration t1, the front-porch portion 2 of the blanking signal, and the back-porch portion 3 of the blanking signal. In conventional television systems the amplitude level of the carrier wave is maintained substantially constant, generally at 75% of maximum carrier amplitude, throughout tion, this practice is departed from in a manner to be described hereinafter. 4

Although the present invention is'applicable to common carrier as well as to alternate carrier television systems, the invention will be described with particular reference to the alternate carrier system of transmission which is described in detail in the copending application of F. J. Bingley, Serial No. 401,533, filed July 8, 1941, and in my copending application, Serial No. 401,494, filed July 8, 1941. The frequency characteristic of the carrier wave here under consideration is illus-' trated in the lower portion of Fig. l and i plotted against frequency deviation in megacycles, i. a, deviation from the normal blanking and video carrier frequency. This normal or assigned carried frequency may be of the order of 60 or 70 'megacycle's. In a conventional alternate carrier.

television system, the carrier frequency'during the blanking intervals (excluding the synchroniz ing intervals to and during the video intervals B is held constant at the assigned video frequency Iv where the deviation is, 01' course, zero. During the synchronizing intervals t1, t1", etc. the carrier frequency is established at a different value 1.: which may differ from the normal carrier frequency by. say. one megacycle. This alternate carrier system of television transmission provides improved synchronizing performance at the receiving station for reasons which are fully described in the last-mentioned applications of Bingley and Smith.

In a 525-line television system the complete line interval has a duration of approximately 63.5 micro-seconds, and this period is divided approximately as follows: interval Al0.2,microseconds, front-porch 2 -1.3 micro-seconds, synchronizing pulse I5.1 micro-seconds, backporch 3--3.8 micro-seconds, and video interval B-53.3 micro-seconds. The signal representations of Fig. l are taken for identical lines in successive frames, and hence are separated in time by a 525-line interval is of substantially one-thirtieth of a second. If now a wave reflecting structure is so situated with respect to the transmitting and receiving antennas that an echo of the carrier wav is received, say, 20 microseconds after the reception of the original signal which travels the direct path, it will be seen that an echo of the synchronizing signal I will arrive during the following video interval B and will beat with the carrier wave for a time equal to the duration of the synchronizing signal I, which in the case under consideration is 5.1 micro-seconds. Consequently if a synchronizing signal I of duration i1 is delayed for a time t4, the synchronizing signal echo will beat with the Video carrier during the portion t2 of the interval B. wher the interval t1 is equal to the interval is. The portion 4 of the signal corresponding to the time interval t2 has been cross-hatched to indicate diagrammatically that this part of the signal is interfered with by the delayed reception of the synchronizing signal I Similarly the echo of the next-illustrated synchronizing signal I will be delayed a like amount t4, and will interfere with the video carrier during the interval 152' as indicated by the cross-hatched area 4'.

As is disclosed on the above-named copending joint application of Bingley and Bradley, the echo images appearing on the screen of the television receiver may be eliminated, so far as the observer is concerned, by transmitting the television signal in a manner such that dark echoes in one frame are followed by corresponding light echoes in the immediately succeedingframe, and vice versa. This may be accomplished by periodically reversing the phase of the carrier wave, as between synchronizing and video intervals, in a sequence such that the phase or polarity of the resulting beat or echo signal is opposite in successive frames. The present invention provides an improved method of effecting the carrier phase reversals or changes. In the preferred embodiment of the present invention the carrier phase reversals (i. e. 180 phase changes) are not obtained directly, but rather through pairs of phase changes of each, and these changes are obtained not by a substantially instantaneous phase change, but by a relatively gradual phase again be returned to frequency .fv.

time successive identical lines, the echo 4' subto 67.5 megacycles, and-that we wish to utilize 2 micro-seconds of the 3.8 micro-second backporch 3 for effecting the 90 carrier phase change by the frequency deviation method of the present invention. The desired phase change c is given by the equation where Edi/2 is the shifted frequency and arc/2 is the original frequency. Now in two micro-seconds the 67.5-megacycle radio frequency carrier will go through 135 cycles. Hence. to effect a 90 phase change requires a frequency shift of 90/135, or two-thirds of a degree per'cycle. This corresponds to a frequency change of or 125 kilocycles. Thus'the frequency may be shifted higher by 125 kilocycles and held for two micro-seconds to produce a phase advance of 90,

or reduced by 125 kilocycles to produce a phase stantially balances out the echo 4.

In the foregoing, specific reference has been made only to the phase change as between identical lines in successive frames. In the preferred embodiment of the invention, for systems using an odd number of lines, the same alternating phase changes are effected in successive lines in time sequence (this is equivalent to'alternate lines in space sequence in interlaced systems).

' It will be seen that in any odd-line system where or lost, and then to return the frequency of oscillation to normal.

The above-described frequency changes which produce the desired phase shifts during the backporch intervals tar and tar are plotted in the lower portion of Fig. l which illustrates the frequency excursions'of the carrier in megacycles. Assume that we have established the normal, or video, carrier frequency fv (where deviation is zero) at 67.5 megacycles. In accordance with the princithe carrier may be shifted to 68.5 megacycles during the synchronizing intervals t1, h. This amounts to a deviation of one megacycle, and es- .tablishes the carrier at the frequency h during the said synchronizing intervals.

At the receiving station the carrier of the delayed synchronizing signal I will beat, or heterotube in a manner which is fully described in the above-mentioned joint application of Bingley and Iii 'ples of alternate carrier television transmission Bradley. The phase or polarity of the echo will be determined by the difference in path length between the directly-received and the reflected signals, which is fixed, and by the relative carrier phase of the video and synchronizing portions of the transmitted signal, which is controllable at the transmitter by means of the frequency deviation method of the invention. During the portion tx of the back-porch interval ta the carrier frequency may be increased 125 kilocycles to the upper phase-shifting level I and held at that frequency until the desired carrier phase ad vance has beenproduced, at which time the carrier'may be returned to frequency fv. During the portion is of the back-porch interval t. the carrier frequency is decreased kilocycles to the lower phase-shifting levels f, and held at that frequency until the desired 90 phase retard has been effected, at which time the carrier may The effect of however, preferred.

in the phase is alternately shifted forward and synchronizing interval, the frame relations described with reference to Fig. 1 will be preserved, and hence echo cancellation will obtain over the entire scanning pattern. It is not necessary that the phase shifting operation take place during the vertical synchronizing intervals, although the proper sequence must be preserved throughout.

In the preferred odd-line embodiment just described, the general appearance of the individual frames, when receiving a strong echo, is substantially as described with reference to Fig; 6 'of the above-mentioned joint application of Bingley and Bradley.

The-invention is, of course, not limited to the specific combination of frequency deviation and interval tx calculated above. Moreover it is not necessary that a rectangular frequency shift characteristic (see Fig. 1) be utilized in the interval tx, and if desireda triangular or parabolic characteristic may be adopted. Furthermore the invention is not limited to the specific 90-degree phaseshift method (which provide carrier phase reversal in two steps). If desired the invention may be utilized to provide the -degree phase shifts directly, these being timed and utilized in a manner taught by the above-mentioned joint application of Bin'gley and. Bradley. Likewise the present invention may be utilized to effect echo cancellation by the polyphase (e. g. threephase) methods described in the said joint application. In a three-phase system echo cancellation may be secured by changing the phase of the carrier oftime successive identical lines by an amount a, a+l2.0, and al20, or in general where nk=360; a small integral value of n is, In its broadest aspect'the present invention should be regarded as one which the described phase advance and phase retard is,

of course, to reverse the character of the echo for identical lines in successive frames. when the reversed echoes are superimposed one ,cn the discloses a method and mean for effecting a desired phase changeby means of frequency deviations which are controlled both as to duration and magnitude. V

Reference is now made to Fig. 2 which shows the frequency response characteristic of a typical television receiver adapted to receive the standard single-side-band transmissions. This response is characterized in that it varies from detector, and a iive to-one-increase in the synchronizing signal output of the detector; this is other, as is done by the eye in viewing quickly the 75 fully explained in the above-mentioned copending application of Smith and copending application Serial No. 401,533 of Bingley. In a similar manner the lesser frequency shifts which occur during the phase-shifting intervals tx, tx', tend to change the amplitude of the signal developed in the receiver. Specifically, a frequency deviation or shift of 0.125 megacycle on either side of the videocarrier frequency Iv will produce a change in developed signal amplitude of 0.125/1.5 or onetwelfth of the full amplitude. Taking into account the fact the normal black level (75% of maximum carrier amplitude) is attenuated 50% (at frequency fv) it follows that'the amplitude variation produced by the 125 kilocycle frequency shift can be substantially eliminated by a compensating carrier amplitude change of approximately I in terms of the peak carrier at the transmitter.

Therefore at the same time that the carrier is shifted upward by 125 kilocycles its amplitude may be cut from the normal black (i. e., blanking) level of 75% to 62.5%, and when the car- 'rier is shifted downward by 125 kilocycles its source of carrier wave oscillations is an oscillator 5 of controllable frequency; Where the oscillator operates at a sub-multiple of the transmitters assigned frequency it may be followed by a frequency multiplier stage]. Following the multiplier stage there may be an amplifier l, a modulated stage 8, a modulator 8, a linear amplifler ill, a sesqui-side-band illter H, and an an- J tenna 12, all of which may be of conventional design. Video, synchronizing. and blanking signais may be supplied from the source l3, through a device i'l whose function will be described in detail hereinafter, to the amplitude modulator Q. The combined video, synchronizing and blanking signal is also supplied to a synchronizing signal separator stage 1-5. whose function is to The circuit of Fig. '3, as thus far described, is

capable of generating the signal of Fig. 1 with the exception of the frequency and amplitude changes occurring during the intervals is and "H of the back-porch portions 3 of the blanking isnot described in detail. Horizontal synchronizing signal from the device It is supplied to a frequency halver stage I8, which supplies a sig-- nal of half the horizontal synchronizing frequency to the sine wave'generator i911, which in tum produces a sine wave signal whose frequency is also half the horizontal synchronizing frequency. The device iS-lfla may comprise, "as will be understood by those skilled in the art,

, a blocking tube oscillator adjusted to fire at separate the synchronizing signals from the combined signal. Synchronizing signals from the separator stage II may then be supplied through a device It, whose function will bedescribed hereinafter, to a frequency shifter ll, the function of which is to shift or deviate the frequency of thewscillator I in accordance with the control signal supplied by the device II. The application" of the synchronizing sisnais to the frequency shifter I'I controls the frequency of, oscillation of the oscillator I in accordance with the'principleaof alternatecarrier transmission described in the above-mentioned copending application of Smith. and copending application Serial No. 401,583 ofBlngley. The frequency shifter I! may be anysuitable device which is adaptedto shift thagfrequency of the oscillator in either direction.in..-response to a control signal. For exampie, various embodiments of such .a device are shown in U. a. Patent No. assume to c; Travis.

every second horizontal synchronizing pulse; this blocking tube oscillator may be followed by one oiymore resonant circuits tuned to the desired half-frequency to produce an output signal of approximately sine wave shape. This latter signal may then be supplied to the square wave generator 20 yhich produces a substantially square wave signal of half the horizontal'synchronizing frequency. The square wave output of the unit 20 is supplied to the narrow pulse generator 2i, which so utilizes the square wave as to produce a wave of the type shown at 22, which comprises spaced alternately positive andnegative narrow pulses, each pulse having a duration of approximately two micro-seconds to correspond in duration and timing to, the intervals tx0f,F1g. l. The signal 22, for reasons which will become apparent, may be referred to as the "echo-cancellation signal. A- satisfactory method for generating such alternating pulses of predetermined width and timing is fully described in U. S. Patent No. 2,171,536 to F. J. Bingley (see especially Figs. 4 and 21). Devices suitable for use in the unit lO-lta are also described in the said Bingley patent.

' The alternating narrow pulse 22, derived from the narrow pulse generator 2 I is supplied through the blanking amplifier 23 to the signal combiningampliiler it where it is combined with the synchronizing signal from the unit I5. The com bined signal 25 is then applied to the frequency shifter stage llwhere it controls 'the frequency of the oscillator l in accordance with'the frequency characteristic illustrated in the lower portion of Fig. 1. Preferably a gain control device 20 is inserted between the device 23 and the signal combining amplifier [6' so that the relative amplitude of the signals 22 and 24 may be controlled to provide the proper relation between 1- on the one hand. and f. and'f. on the other (see Fig. l).

As is indicated in the above-mentioned copending Joint application of Bingley and Bradley; it is preferred that the phase changing 0p-- srations be suspended during the vertical synchronizing intervals. In the present invention from the" unit "II are supplied to the-"device '21 which generates a blanking signal of'such duration and magnitude that the gain of the said amplifier is periodically reduced substantially to zero, thereby to interrupt the supply of'the signal 22 to the signal combining amplifier l8 during the vertical synchronizing intervals. Blanking amplifiers are, of course, well known to those skilled in the art and consequently a detailed description of this device is not deemed neces-' during the same intervals, the signal 22 is supplied, by way of the signal channel 28, to the signal combining amplifier H where it is combined withthe video, synchronizing, and blanking signal and thence supplied to the modulator stage 9 which, in combination with the modulated stage 8 effects the amplitude modulation of the carrier wave in accordance with the principles already described with reference to Fig. l. Preferably the signal channel 28 includes a gain control device 28 toenable the amplitude of the alternating narrow pulse signal 22 to be adjusted to a point where the presence of this signal Just compensates for the effect of the frequency shifts which occur during the back-porch intervals ta, ta", etc.

Therelative timing and general relation between the several signalsreferred to with reference toFig. 3 are best understood in connection with the illustration of Fig. 4. In this illustrae tion, no attempt has been made to maintain the identical scales used-in the signal representations of Fig. 1 or 3. The first signal, E13, represents the combined video, synchronizing and blanking signal outputof the source l3. E represents the synchronizing signal'output of the device It. It includes horizontal and vertical synchronizing signal components, the latter not occurring in the interval chosen for illustration. The signal Eu represents the horizontal synchronizing signal which is derived from the separator stage I8 and supplied to the frequency halver l9. E21 is the alternating narrow pulse signal supplied by the narrow pulse generator 2|, and corresponds. to the signal 22 illustrated in Fig. 3. It will be noted that the narrow pulses occur within the backporch intervals of the signal E13, and they may be of substantially two micro-seconds duration. The generation of the alternating "echo cancellation signal" 22, even though'its timing differs from that of the horizontal synchronizing signal of occurrence of the narrow pulses.

. a,soo,o1a

'- q ncy.

a the multiplier is not employed. Howeveritis usually desirable to operate the oscillator at a much lower frequency inasmuch as frequency control circuits are usually more convenie'ntly applied at lower frequency levels. Where a multiplier is used, and the,oscillator is operate d at a sub-multiple of the transmitters as-v signed frequency, the frequency shifts which are effected during the synchronizing intervals t1, t1,

etc., and the'shorter intervals tn, tx', etc., are reduced to l/nth of the deviations referred to with reference to Fig. 1,.where n is the multiplying, factor of the frequency multiplier 6.

In certain of the appended claims mention is madeof the phase-shift method" of echo cancellation; this reference is, of course, to the broad method of echo cancellation disclosed in the above-mentioned joint application of Bingley and Bradley.

Althougn. the invention ,has been described with reference to a specific embodiment, it will be understood that it is capable of expression in other physical forms which will occur to those skilled in theart after reading the foregoing description, and that widealterations and modiflcations may be made within the scope ofthis invention as defined in the appended claims.

I claim:

- 1. In an echo neutralizing system for television transmitter and the like, the method of effecting g a predetermined phase change in a carrier wave of predetermined frequency, which comprises changing the frequency of said carrier wave, confrequency when the desired phase changes have been effected.

3. In a television transmission system, the

-method which comprises periodically changing the frequency of the carrier wave to effect phase shifts in said carrier wave, controlling said frequency changes both as to magnitude and duration to produce phase shifts of predetermined magnitudes, and so establishing the periodicity and magnitudes of said phase shifts that echo The signals E16 and E21 are supplied to the signal combining amplifier l6 which combines the said signals to form the combined signalEm,

which corresponds to the signal 25 illustrated in the frequency of the output signal of the oscillater 5.

In Fig. 3 the frequency multiplier 8 is shown as optionalequipment. If the oscillator t is opcancellation is obtained in accordance with the phase-shift method.

. 4. The method according to claim 3, wherein aid frequency changes are effected during blanking interval portions of the transmitted wave.

. 5. In a television transmission system, the method which comprises periodically advancing the hase of the carrier wave by means of a, carrier frequency deviation sustained for a short time, periodically retarding the phase of said wave 90 by means of a carrier frequency deviation sustained for a short time, and so fixingthe periodicity of saidphase changes that echo cancellation is effected in accordance with the phase-shift method.

6. The method according to claim 5, wherein erated directly at the transmitters assigned freu the said. carrier frequ y deviations are of the periodically advancing the phase of the carrier wave 90 through the agency of a brief. carrier frequency increase, utilizing a portion of other of said back-porch intervals for periodically re-- tarding'the phase of the carrier wave 90 through the agency of a brief carrier frequency decrease,

and so ilxingthe-sequence and periodicity of said-changes that echo cancellation is effected in accordance with the phase-shift method.

' 9. In a television transmission, system, the method which comprises periodically changing the frequency of the carrier wave to, effect predetermined phase shifts therein, simultaneously changing the amplitude of said carrier wave to compensate, in television receivers having slop- Y ingfrepuency response characteristics, .for the amplitude variations introduced by said frequency changes, controlling said frequency changes both asto magnitude and duration to produce phase shifts of predetermined magnitudes, and so establishing the periodicity and magnitudes of said phase shifts that echo cancellation is obtained in accordance with the phase-shift method.

10. The method according to claim 9, wherein said periodic frequency and amplitude changes are interrupted during the vertical synchronizing intervals of the transmitted carrier wave.

11. In a television transmitter, a source of carrier wave oscillations, means responsive to acontroi signal for controlling the frequency of said oscillations, means for generating a control sig- I aseaeis forder'of 125,000 cycles, and the deviation inter- 14, wherein the last-recited means is adapted to produce phase changes each having a magnitude of' substantially 90 degrees. 16. In a television transmitter or the like, a

source of carrier wave oscillations, means including a frequency-deviating device for periodically shifting the phase of said carrier wave for the purpose of effecting echo signal cancellation in accordance with the phase-shift method, and

means operative synchronously with said frequency-deviating device for amplitude-modulating said carrier wave to prevent amplitude transients when said wave is received by a receiving circuit having a sloping frequency-re-- sponse characteristic.

17. A television'transmitter as claimed in claim 16, wherein the recited means are adapted to effect said carrier frequency deviations and said synchronous carrier-amplitude-modulation during intervals other thanthe synchronizing and video intervals.

18. A television transmitter as claimed in claim 16, wherein the recited means, are adapted to effect said carrier frequency deviations and said synchonous carrier-amplitude-modulation dur; ing the back-porch blanking intervals.

19. A television transmitter as claimed in claim 16, wherein the first recited means is adapted to advance the phase of said carrier 90 after alternate synchronizing intervals and to retard the carrier phase 90 after the other synchronizing intervals.

nal which, when applied to said control means,

carrier wave to efiect phase shifts in said wave. said frequency changes being of such magnitude and duration as to produce phase shifts of predetermined magnitude, and. means for so establishing the periodicity and magnitude of said phase shifts that echo cancellation in accordance with the phase-shift method is obtained.

12. A television transmitter as claimed in claim 11, wherein the last-recited means is adapted to produce phase shifts of the order of 360/12. degrees, where n is a small integer.

13. A television transmitter as claimed in claim 11, wherein the last-recited means is adapted to produce carrier phase shifts in the intervals which occur in the periods between the end of the synchronizing intervals and the beginning of the video intervals.

14. In an, echo neutralizing circuit for television transmitters and the like, a source of carrier wave oscillations, means responsive to a control signal for controlling the frequency of said oscillations, means for generating a control signal for application to said control means for I alternately increasing and decreasing said carthatecho cancellation in accordance with the.

phase-shift method is obtained.

. I, 15. A television transmitter as claimed in claim 20. In a television transmitter, a source of carrier wave oscillations, means responsive to a con trol signal for controlling the frequency of said oscillations, a source of synchronizing impulses, means responsive to said impulses for generat ing a control signal adapted to change the frequency of said carrier wave periodically in a manner to eflect echo cancellation phase shifts in said wave, and means for applying said control signal to said frequency-control means.

21. In a television transmitter, a source of carrier wave oscillations, means responsive to control impulses of opposite polarity for we the frequency of said oscillations in either direc= tion, a source of synchronizing impulses, means responsive to said impulses for generating a. pulse signal in which consecutive pulses are of opposite polarity and the pulses are arranged to chest echo cancellation phase shifts of the carrier ire quency, and means for applying said pulse #5... to said frequency-control means. 4

22. In a television transmitter, a'source oi car rier wave oscillations, means responsive to a comtrol signal for controlling the frequency of said oscillations, a source of horizontal and vertical synchronizing impulses, means for deriving horizontal synchronizing impulses from said source, means responsive to the said derived impulses for generating an echo-cancellation control signal,

, means for combining said signal with said synchronizing impulses, and means for applying the resultant signal to saidfrequency control means, thereby to effect frequency shifts of the carrier wave both to' transmit synchronizing intelligence and to effect echo cancellation.

23. In a television transmitter, means for generating a carrier wave of predetermined normal frequency, a source of video, blanking and synchronizing signals, means for amplitude-modulating said carrier wave with said signals, means for deriving synchronizing signals only from said source, means for further deriving horizontal synchronizing impulses only,rmeans operable bysaid last-named irnpulsesto generate an echo cancellation signal. means tor combining said derived synchronizing signals with said echo cancellation signal, and a frequency shifter operable by said combined signals toshiit the frequency 5' of the carrier wave v j 24. In a television transmitter, a source of carrier wave oscillations. means responsive to a conoscillations; means for generating a control signal adapted to change the frequency or said carrier wave periodically-in a manner to eflect echo cancellation phase shiits'in said wave, and means ior applying said control signal to said frequency- DAVI D B. SMITH.

" control means. 

